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Abstract

The local perturbation of a diffraction-limited spot by a nanometer sized gold tip in a popular apertureless scanning near-field optical microscopy (ASNOM) configuration is reproduced through topography changes in a photoresponsive polymer. Our method relies on the observation of the photochemical migration of azobenzene molecules grafted to a polymer placed beneath the tip. A local molecular displacement has been shown to be activated by a gold tip as a consequence of the lateral surface charge density present at the edges of the tip’s end, resulting from a strong near-field depolarization predicted by theory.

Figures (4)

Fig. 1. (a) Experimental arrangement for recording the local perturbation induced in a diffraction-limited spot by an ASNOM probe. Inset: SEM image of a gold tip (b) isomerization of the azobenzenic molecule.

Fig. 2. AFM images of the polymer surface taken after the exposure. The white arrows correspond to the incident polarization (a), (b): exposure without any tip. Note that there is no change in images (a) or (b) in the presence of a glass tip. Inset in (a): array of dots written under the same condition as for (a). (c)-(f): exposure with a gold tip placed in the focal spot. (e) and (f) are smaller scan areas of the focal regions (c) and (d). The dashed circle in Figs. 2(c)-(f) highlight the near-field response of the polymer when the tip is present.

Fig. 3. Calculated amplitude of the different field components on the polymer surface beneath a 40 nm radius tip. (a) geometry of the problem. (b) to (d): case of the gold tip. (e) to (g): case of the glass tip. The size of the calculated images is 300×250 nm2.

Fig. 4. (a) Experimental topography profile along the pattern of Fig. 2(d) without the presence of the tip. (b) Experimental topography profile in the presence of the metal tip extracted from Fig. 2(f). The red single and double solid arrows represent the different orientation of the electric field components Ex and Ez, respectively. The dotted blue arrows indicate the corresponding response of the polymer to the electric field components.